Economic exploitation of non fossil and non nuclear heat sources for power generation is dependent on the amount of heat available, its temperature and a size of power plant that can justify its erection costs as well as its operational costs. In this regard, the lower the heat source temperature, the larger the amount of heat (and consequently the bigger the heat exchangers) required for a given power output. As a consequence, the cost of the power plant per kW will be higher. In the case of geothermal energy for example, geothermal fields comprising wells producing low-temperature geothermal fluid cannot be efficiently and economically exploited to operate power plants, since the lower the enthalpy of the fluid, the more expensive the power plant becomes for a given capacity. Due to this, geothermal wells whose drilling costs are substantial are not usually exploited in such geothermal fluids. In addition, the temperature of the geothermal fluid produced by operating geothermal wells may drop after some years of operation causing a reduction of the output of the geothermal power plant as well as its possible derating by the electric utility.
Most low-temperature heat sources are in the form of sensible heat which is most efficiently used for preheating in Rankine cycle power plants. Their use for vaporizing the motive fluid at constant temperature in the power cycle is not effective (see FIG. 1). In this respect, organic fluids require more pre-heating than water/steam and thus more effectively utilize the sensible heat.
As a consequence, many low temperature heat source fluids cannot be effectively used.
As far as solar power plants are concerned, their cost is very dependent on the solar collector cost or solar collector size per kW. Furthermore, in order for solar power plants to provide continuous power, expensive storage (requiring additional solar collectors) and/or fossil fuel needs to be used.
Some methods have been devised so far to exploit low heat and thus low input from the geothermal sources in a more efficient manner. U.S. Pat. Nos. 4,578,953 and 4,700,543, the disclosures of which are hereby incorporated by reference, of Krieger and Moritz describe a plurality of independent, closed Rankine cycle power plants, each of which has a vaporizer, operated by serially applying a medium or low temperature source fluid to the vaporizers of the power plants for producing heat depleted source fluid. A pre-heater is provided for each vaporizer; and the heat depleted source fluid is applied to all of the pre-heaters in parallel. The heat depleted source fluid thus serves to heat the operating fluid to the vaporization temperature, while the source fluid applied to the vaporizers supplies the latent heat of vaporization to the operating fluid of the power plant.
Attempts in the past have been made to combine geothermal and solar energy but without addressing the issue of optimization of the specific characteristics of the respective heat sources. For instance, U.S. Pat. No. 3,950,949 discloses a method for operating a vapor cycle engine wherein a vaporizable fluid is circulated in a closed loop through a first heat source, a second heat source, an expansion vapor engine, a condenser, and back to the first heat source. The method of operation comprises the steps of heating the fluid in the first heat source to a relatively low temperature t to vaporize at least a major portion of the fluid, super-heating the fluid in the second heat source to a temperature T, supplying the fluid to the expansion vapor engine for producing power and then exhausting the expanded fluid from the engine at a temperature T2 which is greater than the temperature t. This patent describes the combination of geothermal heat and solar heat in a manner which is not efficient, since the aim is to superheat the circulated fluid. To date, no practical application of this method has been made.
Another attempt to integrate geothermal and solar energy is described in U.S. Pat. No. 4,099,381 where an energy transport and conversion system is provided for conducting geothermal energy over extended distances avoiding the lose of heat by use of solar energy in order to permit efficient conversion of geothermal energy at a central thermal power station. However, the system described in this patent is impractical and expensive and, furthermore, is designed to prevent loss of existing geothermal energy, and not to exploit low enthalpy geothermal energy that would otherwise remain unexploited.
It is an object of the present invention to provide a method for exploiting low-medium temperature geothermal fluid produced by geothermal wells so that their economical exploitation can be achieved.
It is another object of the invention to provide a method for exploiting such low-medium quality geothermal fluid produced by geothermal wells by combining them with the use of solar energy in an efficient and economical manner.
It is a further object of the invention to provide a system which enables the method of the invention to be carried out.
Other objects and advantages of the invention will become apparent as the description proceeds.